Java 8, 17 bytes

s->1024-s.sum()/2

Port of @LuisFelipeDeJesusMunoz' JavaScript answer.

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Explanation:

s-> // Method with IntStream parameter and int return-type
 1024- // Return 1024, minus:
 s.sum() // The sum of the input-IntStream
 /2 // Divided by 2

Brain-Flak, 72 bytes

()(<([()()])>())([]((((((((()()()())))))))))

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() # Sum entire stack
 ( ) # Push:
 < >() # The number of times you can...
 ([()()]) # Subtract 2 before reaching 0

([] ) # Subtract that from...
 ((((((((()()()())))))))) # 1024

05AB1E, 5 bytes

O;žBα

Port of @LuisFelipeDeJesusMunoz' JavaScript answer.

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Explanation:

O # Sum of the (implicit) input-list
 ; # Halved
 žB # Push 1024
 α # Absolute difference between the two (and output implicitly)

Python 2, 22 bytes

lambda x:4**5-sum(x)/2

Y'know, I just realized that 4**5 is the same length as 1024.

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JavaScript (Node.js), 25 bytes

Based on @Shaggys' comment and answer

-3 bytes from @Arnauld =D

_=>1024-eval(_.join`+`)/2

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MathGolf, 6 5 bytes

Σc/◘+

First MathGolf answer. :)

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Explanation:

Σ # Sum of the (implicit) input-list
 c/ # Divide it by -2
 ◘+ # Add 1024 to it (and output implicitly)

APL (Dyalog Unicode), 10 bytes

2÷⍨2048-+/

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Ruby, 17 bytes

->l1024-l.sum/2

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JavaScript, 28 bytes

a=>a.map(n=>x-=n/2,x=1024)|x

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J, 10 bytes

2048-:@-+/

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Alternative:

J, 10 bytes

1024-1#.-:

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Jelly, 7 6 bytes

HSạ⁽¡ç

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Explanation:

HSạ⁽¡ç Example input: [4,16,64]
H Halve input. [2, 8, 32]
 S Sum. 42
 ⁽¡ç Number 1024.
 ạ Difference. 982

_{-1 byte by using a base-250 number}

Catholicon, 5 bytes

-`L̇½Ṗ

Explanation:

- subtract
 `L̇ 1024
 from
 ½ half of the
 Ṗ sum [of the input]

TI-Basic, 8 bytes

4^5-.5sum(Ans

Japt, 7 6 bytes

xz nH²

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 z :(Floor) divide each by 2
x :Reduce by addition
 n :Subtract from
 H : 32
 ² : Squared

JavaScript (Node.js), 33 bytes

a=>!a.map(n=>x-=n/2,x=1024)>x?0:x

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Why you don't do on [1024,1024,1024]?

R, 17 bytes

4^5-sum(scan())/2

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TeaScript, 11 bytes

4p5)-(xx)/2

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Stax, 6 bytes

ë3i─♠÷

Run and debug it

perl -aE, 27 bytes

$"=$:;say eval"(2048-@F)/2"

This reads a line with numbers (whitespace separated) from STDIN, and writes the answer to STDOUT.

What it does is subtract all the numbers from the input from 2048, and it divides the remainder by 2. The -a switch puts the in the array @F (one number per element). If we interpolate an array into a string (which is what we are doing here), perl puts the value of $" between the elements. The little used variable $: is be default n-; and since white space between tokens is ignored, the result is subtracting all the numbers from 2048. The eval does the calculation.

Perl 6, 12 bytes

1024-*.sum/2

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Anonymous Whatever lambda that takes a list and returns a number.